Large electrical generators or motors can include sliprings or commutators and associated brushes, via which the field current or drive current is supplied. These brushes can be manufactured from carbon and graphite materials and are subject to relatively high wear. For smooth operation of an electrical machine with brushes such as these, it is desirable to monitor the brushes, in particular the brush wear, as continuously as possible.
U.S. Pat. Nos. 4,950,933 and 4,739,208 have disclosed apparatuses in order to monitor the length of slipring brushes for a predetermined minimum value. When the predetermined minimum value is undershot, this mechanically or magnetically initiates a closing or opening process of an electrical contact. The closing or opening of the electrical contact is detected, and is indicated by a signal. It is therefore possible to determine, in the form of a length statement, whether the brushes have or have not undershot the length limit value.
U.S. Pat. No. 5,870,026 discloses an apparatus by which the instantaneous, still remaining length of a brush can be output as an analog value. For this purpose, a resistance element is embedded in the brush and is shortened as it wears away. The resistance, which changes as it wears away, is used as a measure of the length of the brush.
However, in these known apparatuses, in some cases, only a local indication of the fault state, that is to say of a worn-out brush, is provided. Since the brush apparatuses of electrical machines can be in a closed form, this indication can easily be overlooked, or cannot be monitored so easily by maintenance personnel.
In some cases, remote indication is admittedly also made possible, although electrical signal connections are then used for this purpose via cables from the outside to the brush apparatus. Because of the generally high electrical voltage and the high potential energies which exist in electrical machines, for example in the case of an electrical generator, when in the energized state, this results, however, in a major potential danger. Measures to cope with this potential change are generally quite complex.